When two peaks corresponding to distinct components are very close and are not fully separate in a chromatogram, a comparative chromatogram method is used. In the comparative chromatogram method, an index P is calculated by the following equation (1) using absorbance values A(.lambda..sub.1), A(.lambda..sub.2) and background values A.sub.B (.lambda..sub.1), A.sub.B (.lambda..sub.2) at two preset wavelengths .lambda..sub.1 and .lambda..sub.2 in every absorbance spectrum detected repeatedly on the mobile phase effluent from the column. EQU P={A(.lambda..sub.1)-A.sub.B (.lambda..sub.1)}/ {.vertline.A(.lambda..sub.1)-A.sub.B (.lambda..sub.1).vertline..sup.2 +.vertline.A(.lambda..sub.2) -A.sub.B (.lambda..sub.2).vertline..sup.2 }.sup.1/2. . . (1)
The index P has the following meaning. Let us introduce a vector aa having components (a.sub.1,a.sub.2) which are given as follows. EQU A(.lambda..sub.1)-A.sub.B (.lambda..sub.1)=a.sub.1 EQU A(.lambda..sub.2)-A.sub.B (.lambda..sub.2)=a.sub.2
Using the vector aa, the equation (1) is rewritten as EQU P=a.sub.1 /.vertline.aa.vertline.
This means that the index P represents the cosine of the angle .theta. (i.e., cos.theta.) between the vector aa(a.sub.1, a.sub.2) and a vector (a.sub.1, 0) as shown in FIG. 7.
When the mobile phase (or solvent) alone is flowing out of the column, A(.lambda..sub.1)=A.sub.B (.lambda..sub.1) and A(.lambda..sub.2)=A.sub.B (.lambda..sub.2) because what is detected is the background only. Thus, a.sub.1 =a.sub.2 =0 and the vector aa is a zero vector. When a peak in a chromatogram is a simple peak corresponding to a single component as shown at the left of the top .lambda..sub.1 -.lambda..sub.2 curves of FIG. 8, the direction of the vector aa is unchanged though the length thereof varies because the ratio of values of a.sub.1 and a.sub.2 is constant. That is, the vector aa moves within the linear region P.sub.1 shown in FIG. 7, and the value of index P (=cos.theta.) is constant as shown at the left of the bottom curve of FIG. 8.
When, on the other hand, a peak is complex and corresponds to plural different components as shown at the right of the top .lambda..sub.1 -.lambda..sub.2 curves of FIG. 8, the ratio of the absorbance values at the wavelengths .lambda..sub.1 and .lambda..sub.2 changes with respect to time. Thus the vector aa moves two-dimensionally within a region P.sub.2 as shown in FIG. 7, and the value of index P changes as shown at the right of the bottom curve of FIG. 8.
That is, in the comparative chromatogram method, a peak is judged to be simple or complex by detecting whether the value of index P changes with respect to time.
However, the comparative chromatogram method has following drawbacks.
(i) The value of index P cannot be absolutely constant since, in equation (1), the absorbance at the preset two wavelengths A(.lambda..sub.1), A(.lambda..sub.2) are subtracted by respective background values A.sub.B (.lambda..sub.1), A.sub.B (.lambda..sub.2), and every measured absorbance generally includes incessant fluctuation or noise. Thus, conventionally, an operator empirically sets a tolerance for the index P depending on the apparatus, and it is assumed that variation in the value of index P within the tolerance is due to noise.
(ii) It is necessary to determine beforehand in chromatography a critical concentration (percentage) of impurities which can be definitely detected. In the comparative chromatogram method, it is difficult to determine a definite critical value beforehand because of the reason above.
(iii) When an object component is desired to be separated from a sample, a column chromatograph is used. If an impurity having a retention time close to the object component is included in the sample, the separation is difficult. But if such a period in the retention time that includes the impurity is known, the separation becomes possible. Conventional methods cannot teach such a period.